Rapid solidification route aluminium alloys containing chromium

Details Rapid solidification route aluminium alloys containing chromium Rapid solidification route aluminium alloys containing chromium

1989-04-13

1991-09-17

Dean, R.

Metal treatment

Stock

Aluminum base

420522, C22C 2100

Patent

active

050492110

DESCRIPTION:

BRIEF SUMMARY
This invention relates to aluminium based alloys containing chromium, made by the rapid solidification rate (RSR) route.
Conventional high strength wrought ingot aluminium alloys have limited thermal stability at temperatures above about 150.degree. C. because of coarsening of the precipitates on which their high strength depends. This precipitate coarsening stems from a combination of high diffusivity and appreciable equilibrium solid solubility in aluminium of the alloying elements usually employed (such has zinc, copper, magnesium, silicon and latterly lithium) and significant interfacial energy of the precipitate/matrix interface at these relatively elevated temperatures.
The desirability of adopting other alloying elements to confer improved high temperature stability for high strength wrought ingot aluminium alloys is frustrated by the limited maximum equilibrium solid solubility of elements other than those mentioned above. Such limited solid solubility leads to the formation of coarse embrittling intermetallic compounds on solidification via the conventional ingot route.
It would be desirable to have a high strength aluminium alloy with better high temperature stability than that afforded by known ingot route materials. The RSR route offers a way of enlarging the field of alloying elements for it offers a way of circumventing equilibrium solid solubility limitations and enables a way of producing aluminium based alloys with a higher volume fraction and better dispersion of suitable elements or intermetallic compounds. A fine dispersion of such intermetallics which is also evenly distributed avoids the undesirable embrittlement experienced when these alloying elements become segregated in production of materials via the ingot route. Moreover the intermetallics formed by suitable elements can possess a high resistance to coarsening (leading to enhanced thermal stability) because they have a high melting point coupled with a low diffusivity and solubility in solid aluminium at the temperatures in question.
Various RSR routes are well established. They possess in common the imposition of a high cooling rate on an alloy from the liquid or vapour phase, usually from the liquid phase. RSR methods such as melting spraying, chill methods and weld methods are described in some depth in Rapid Solidification of Metals and Alloys by H. Jones (published as Monograph No 8 by The Institution of Metallurgists) and in many other texts. The various RSR methods differ from one another in their abilities in regard to control of cooling rate. The degree of dispersed refinement and the extension of solid solubility are dependent on the rate of cooling from the melt.
Previous workers have sought to use RSR methods to produce aluminium alloys having good strength coupled with improved thermal stability. Binary alloys which have been investigated include aluminium-iron, aluminium-chromium, aluminium-manganese and aluminium-zirconium. U.S. Pat. No. 4,347,076 claims a vast range of compositions within the scope of aluminium with 5/16 weight percent of one or more of iron chromium nickel cobalt manganese vanadium titanium zirconium molybdenum tungsten and boron; although few of these combinations are examplified other than aluminium-iron bases ones.
Two drawbacks of basing developments on systems of the widely explored aluminium-iron type are that conditions of rapid solidification required to generate segregation-free and/or extended solid solutions approach the limits of standard rapid solidification processing and that fine-scale decomposition within these solid solutions puts them into their hardest condition making consolidation exceptionally difficult.
The need to aid processability by relaxing both of these limitations led to the exploration of the potential of the aluminium-zirconium, aluminium-chromium and aluminium-manganese systems and their combinations as alternative bases for alloy development. All three systems start to exhibit extension of solid solubility even under chill-casting conditions of rapid solidifica

REFERENCES:
patent: 4347076 (1982-08-01), Ray et al.

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